Effect of different water supply regimes on growth and size hierarchy in spring wheat populations under mulched with clear plastic film

Abstract

This study was conducted to determine the effect of water supply regimes and plastic film mulching on the harvest index (HI), reproductive allocation (RA) and size hierarchy in spring wheat (Triticum aestivum L.) populations, and to explore their mechanism in relation to size hierarchy and life-history strategies.

The grain yield, biological yield (aboveground biomass), HI and RA of spring wheat decreased significantly (p < 0.001) along the water control gradient (irrigative amount decreased 132 → 66 → 0 mm) either mulching or non-mulching, size hierarchy (as measured by the Gini coefficient (G) of aboveground biomass per plant) always increased. HI and RA in mulching treatment were significantly lower than non-mulching (p < 0.05). Meanwhile, the number and weight of barren shoots and the ratio of barren shoot biomass to total shoot biomass were significantly greater (p < 0.05) in mulched populations than non-mulched controls both at booting, flowering and ripening stages. RA and HI were both negatively correlated to average G of the populations. These results suggested that size hierarchies in spring wheat populations are closely correlated with the water regime in the field, and that under greater drought stress there are relatively smaller plants with lower HI (size-dependent reproductive allocation). Size hierarchy is an index of competitive status in plant populations under stress environments. Agriculturally, greater size hierarchy may result in growth redundancy, which is detrimental to reproductive allocation and consequently, grain yield. The results support the view that stand uniformity in field crops is an important mechanism for increasing grain yield.

From tilling to ripening stages, the tendency of Gini coefficient (G) shows obvious differences between mulched populations and non-mulched controls. At booting and flowering, the G was significantly higher in mulched populations than non-mulched controls, and it was just contrary at ripening.

Appreciable growth redundancy occurred in spring wheat populations mulched with plastic film, which may result from the exacerbated interplant competition and self-thinning. Thus, spring wheat cultivation with plastic film mulching does not always mean high-efficiency, although there is a remarkable increase in grain yields.

Introduction

Most plant populations consist of relatively few large individuals and many small ones, and the few large individuals account for most of biomass of the population (Obeid et al., 1967, Weiner, 1985, Weiner and Thomas, 1986, Benjamin and Hardwick, 1986). This distribution of biomass has been called “size hierarchies” or “size inequalities” (Weiner and Solbrig, 1984). Due to modem breeding and intensive crop management practices (e.g. genetically uniform pure stands, weed-free, irrigation and fertilizer use, controlled planting date, depth and density), size hierarchies in even-aged plant result mainly from uneven interplant competition within populations (Weiner, 1985, Weiner and Thomas, 1986, Bonan, 1988, Bonan, 1991, Wu and Wang, 1999, Weiner et al., 2001). The ecological and evolutionary significance of size hierarchies is enormous (Harper, 1977, Solbrig, 1980, Wu and Wang, 1999, Xin et al., 1998). For example, the smallest plants suffer density-dependent mortality, and within a population, size appears to be correlated with fitness (Solbrig, 1980, Weiner, 1985).

The arid and semi-arid regions contribute to about 52.5% of the total territory in China. In the region, heat and light is sufficient, Water is principal limit factor in crop production (Wei and Zhao, 1995). There are two main methods to improve water condition of crop growth in the region: mulching and irrigation using rainwater-harvesting.

Crop production in semiarid areas is generally dependent on rainfall, so soil moisture conservation is vital for grain production (Zhao, 1996, Li et al., 1999). Mulching of soil may reduce water loss through evaporation, and therefore may increase water available to plants and grain yield of crops (Cantero-Martinez et al., 1995, O’Leary and Connor, 1997, Li et al., 1999). Studies on vegetable crops have demonstrated that mulches provide several benefits to crop production through soil and water conservation, improved soil physical and chemical properties, and enhanced soil biological activity (Unger, 1975, Tindall et al., 1991). In China's Loess Plateau, plastic film mulching has been utilized in the cultivation of corn (Zea mays L.), cotton (Gossypium hirsutum L.), vegetables and fruit (Zhang and Ma, 1994, Han and Wan, 1995). Recent years this technique has also been introduced in spring wheat. A few studies have been conducted to determine the effect of plastic film mulches on the grain yield potential of spring wheat in the semi-arid regions of Northwest China (Li and Lan, 1995, Li et al., 1999, Du et al., 1999). These studies have shown that mulching increases the number of tillers and grain yield of spring wheat, and mulched spring wheat had a faster seedling emergence and greater dry matter production. Unfortunately, no detailed study has been conducted to describe differences in reproductive allocation between mulched and non-mulched spring wheat populations. This question is critical for determining whether a spring wheat population makes more efficient use of resources under plastic film mulched conditions. In addition, spring wheat plants normally produce tillers, many of which do not survive to produce grain-bearing spikes. Increasing the percentage of tillers that survive has been hypothesized as an approach for increasing yield of wheat plants. But some researchers suggested that as high as 50–70% abortive tillers in certain wheat cultivars (Dewey and Albrechtsen, 1985) is a higher growth redundancy for spring wheat population for higher grain yield (Donald, 1968, Kirby and Jones, 1977).

In recent years, rainwater-harvesting technology has been introduced into dryland agricultural in North China, which improves water availability to the crops (Li et al., 1995, Zhao et al., 1995) and made water control (WC) turning into reality in this areas.

Some important agronomic characters of spring wheat (such as plant height, spike length, grain number per spike, and grain yield) grown along the water control gradient showed that the variation was extensive (Zhu and Shang, 1988). Zi-Zhen et al. (2004) reported that grain yield, spike length, fertile spikelets per spike and grain number per spike increased significantly with irrigation (90 mm) than without irrigation. Among them, grain yield increased 111.7%, spike length increased 54.2%, fertile spikelets per spike increased 71.1% and grain number per spike increased 98.1%. Changing resource pools (e.g. water or nutrient availability) may affect the distribution of biomass (Weiner, 1985, Schmitt et al., 1986, Bonan, 1988, Morse and Bazzaz, 1994, Xin et al., 1998, Wu and Wang, 1999, Duan and Zhao, 1996). Studies of spring wheat populations grown under water limiting conditions showed that the degree of size hierarchies increased with water deficits (Duan and Zhao, 1996, Xin et al., 1998, Wu and Wang, 1999).

The purpose of this study was to investigate (1) whether there was any difference in plant size hierarchy and distributions of spring wheat populations along water control gradient. (2) Whether there was any difference in plant size hierarchy and distribution of spring wheat populations between plastic film mulching and non-mulching. (3) To explore the relationship between size hierarchy and grain yield of spring wheat. (4) To determine the effects of water on the degrees of growth redundancy (increased percentage of abortive tillers and decreased reproductive allocation) in spring wheat population and to explore its mechanism in relation to size hierarchy and life history strategies.